Noise limiter circuits



Nov. 10, 1942. w P. BOLLINGER ET AL 2,301,607

NOISE LIMITEE CIRCUITS Filed July 1, 1941 f4 Mgg/Ww Za /7 :i if@ L f Patented Nov. 10, 1942A NOISE LIlWITER CIRCUITS Waldon P. Bollinger, Haddonield, and Nathaniel I. Korman, Camden, N. J., assgnors 4to Radio Corporation of America, a corporation of Delaware Application July 1,

4 Claims.

This invention relates to noise limiter circuits for signal receiving systems and the like, of the type comprising a rectifier connected with the output circuit of. a signal detector to cut off the signal output therefrom in response to signal peaks and noise voltages above a predetermined value or percentage modulation, and provided with a control voltage from the same source as the detected signal, whereby automatic level setting is obtained,

Noise limiter circuits of this type are represented generally by the U. S. Patent to Kaar 2,207,587. In that patent, however, and as found in other noise limiter systems, the limiter control voltage is derived from the detector output load resistor along with the output signal, and comprises a portion of the available signal voltage in the detector output resistor.

It is an object of this invention, to provide an improved noise limiter circuit wherein automatic level setting control potential or bias voltage is obtained from the same source as the detected signal, as is desirable, without depriving th'e signal output circuit of any of the available detected signal voltage.

It is a further object of this invention, to provide an improved noise limiter circuit comprising a diode rectifier as a demodulator, a limiterdiode rectifier connected with the output circuit of the demodulator, and a voltage-doubler connection from the demodulator for applying a. control potential to the limiter, whereby the full output from the demodulator may be obtained together with automatic operation of the limiter in response to control voltages derived from the same source as the detected signal.

It is a still further object of this invention to provide a combined demodulator for signal voltages, and voltage-doubler for control potentials, in connection with a noise limiter rectier of the diode type, wherein full output voltage may be obtained for th'e detected signal and in which a coupling resistor for the limiter-diode rectifier may be utilized as a volume control device, thereby reducing the effective load on the detector or demodulatcr output resistor.

It is also a still further object of this invention, to provide a demodulator and limiter circuit having an improved arrangement Vof circuit elements, and voltage-doubling means connected therewith whereby the full signal output potential, effective limiter-control potentials, and automatic-volume-control potentials vmay be obtained.

The invention will, however, be further under-i 1941,*seria1 No. 400,616

stood from the following description when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of a noise limiter circuit including the audioV irequency detector of a signal receiving channel embodying the invention; and

Figure 2 is a similar Schematic circuit diagram showing a modiiication of the circuit of Figure 1, embodying the invention in connection, with the second detector of a radio receiver. v

Referring to Fig.` 1, 5 is a diode rectiiier providing a signal demodulator having an anode 6 connected to the high potential terminal .'l-of a signal input circuit 8 and having a cathode 9 connected through a diode-rectier output resistor lll-II with the low potential terminal l2 of the signal input circuit 8. Th'e cathode 9 is connected to chassis or ground as indicated at I3 and signals are applied to the detector circuitl through a suitable coupling transformer I4 from input leads indicated at l5. The latter may be intermediate frequency output circuit leads in the usual superheterodyne receiver,

The output resistor l--ll for the demodul'ator or detector 5 is provided with the usual high' or intermediate frequency bypass capacitor l6` in shunt therewith. An output tap I1 may also be provided.

A Voltage-doubler diode rectiiier 2B of the same type as that provided for the detector 5r is connected back-to-back with the latter through a voltage-doubler resistor 2l having a shunt capacitor 22, the arrangement being such that when a carrier wave is received throughthe. circuit 8, the negative Voltage at the output terminal 23 is substantially twice the detector output negative voltage at the terminal 24, both with respectto cathode or ground I3.

With this arrangement, the full output volt-` age available from the detector 5 across theload resistor lU--ll may be applied to a utilization circuit such as the input` circuit 26 of an audio frequency amplii'lerZl, while at the Sametime permitting automatic level setting operation ci a noise limiter in the output connection. y n

The noise limiter is indicated at 28 and coi v prises a diode rectiiier having an anode electrode 29 connected with the terminal 24 and a `cathode 30 connected through a coupling resistor 3| to. the output terminal 32 of an audio frequency filter connected with the terminal 23 `and com-` prising a series resistor 33 and a by-pass capacitor 34 to chassis or ground.

The anode electrode 29, may be connected to the junction of resistors I and II at the terminal I1 if less output and greater filtering is desired.

In response to a received carrier wave, a negto in connection with the circuit 2S. In the presv ent system, a coupling capacitor 38 is provided in the connection to isolate the circuit 2B from the control potential in the doubler circuit. The coupling resistor 3| is connected at its opposite end with the bias or control potential supply terminal 32 which is by-passed at audio frequency by the capacitor 34. Thus it operates in series with the by-pass capacitor 34 as part of a signal output potentiometer in shunt with the detector output resistor IIJ--I I, and as a filter element to conduct the control potential to the limiter cathode 30.

Referring to Fig. 2, a circuit arrangement similar to that of Fig. 1 is shown in which, however, the limiter-diode rectifier is connected in shunt relation to the output circuit rather than in series therewith, and the three diode rectiers are combined in two tubes along with a fourth diode rectifier which provides automatic-volume-ccntrol potentials. This is a preferred tube arrangement for a limiter system embodying the invention, since a minimum number of tubes is required. It will be noted that the amplifier is also combined with two of the rectifier devices in the same envelope. This reduces the total number of tubes from four to two. I

'I'he tuned signal-input circuit 8, the coupling transformer I4 and the signal input circuit I5 are provided to supply signals to the detector diode-rectifier, comprising an anode 4|] and a cathode 4I, connected across the circuit 8 in series with a detector output load resistor 42. The latter is provided with a by-pass capacitor 43, and the cathode 4I is grounded as indicated at 44. The full signal output from the detector is taken from the high potential end of the output resistor 42 at a terminal 45 through a coupling capacitor 46 and an output potentiometer 41 having a contact 48. The latter is connected through a lead 49 with the control grid 5U of an amplifier associated with the cathode 4I. The output anode I of the amplifier is coupled to an output circuit 52 through a suitable coupling capacitor 53.

The input circuit 8 is also coupled through a capacitor 55 with the anode 56 of a second diode rectifier associated with the common cathode 4I; This rectifier is shunt-connected with an output resistor 51 to which is connected an AVC circuit 58 through a filter resistor 59. Thus the input circuit 8 supplies both the detector 40--4I and the AVC diode rectifier 55-4I.

'I'he output or volume control potentiometer 41 is provided with a bias-voltage circuit lead 60 and with an audio frequency connection to groundthrough a by-pass capacitor 6I. A. usual 75 tone compensation circuit 62 may be provided in connection with this potentiometer.

The voltage doubler comprises a cathode 65 connected to the high potential terminal of the input circuit 8 as indicated at B6, and an anode 61 connected to ground and the cathode 4I through a voltage-doubler resistor 58 in parallel with which is provided a storage or doubler capacitor 69. The arrangement is such that the negative voltage at the doubler output terminal indicated at 10, is substantially twice that at the output terminal 1I of the detector load resistor 42. The cathode 12 of the limiter diode is then connected with the terminal 1I while the anode 13 is connected through a lter resistor 14 to the terminal 1U, thereby causing the limiter-diode rectifier 12-13 to remain non-conducting, since the anode 13 is more negative than the cathode in the presence of a carrier wave, and in the presence of signal voltages below a predetermined percentage modulation or peak value.V

When a condition of excessive noise exists or signal peak amplitudes exceed predetermined limits, the anode 13 is maintained at the existing negative voltage preceding the peak or pulse by the filter resistor 14 and theby-pass capacitor 15 connected between the resistor and ground,

while the cathode follows the variation in amplitude of the terminal 1I. The time constant of the resistor 14 and the capacitor 15 is such that the limiter diode becomes conducting on the peaks, thus cutting 01T peak signals and noise voltages above a predetermined percentage modulation by reason of the shunting action of the circuit formed by the diode 12-13, in series with the capacitor 15 which effectively places it in shunt with the detector output resistor 42.

The circuit constants provided in the detector circuit are substantially conventional. However, the output load resistor at 41 should be of relatively high resistance in order that the load across the output resistor 42 in the diode or demodulator circuit may be relatively light. The resistor 68 is of the order of several megohms, such as 3 megohms, for example, because a low resistance would result in an excessive load on the secondary of the IF transformer I4, that is, across the circuit 8. The resistor 14 is also of the order of several megohms because a low resistance would prevent the voltage-doubler diode -61 from following the modulation on the signals applied to the input circuit 8. The capacitor 69 is of the order of -70 mmfd, as a high value of capacitance here would prevent the voltage doubler diode from following the modulation and a low value of capacitance would not give adequate bypassing for the IF or high frequency component of the rectified signal. The filter and by-pass capacitor 15 may be of the order of .01 mmfd. for the reason that a small capacitance value at this point would not lterthe audio-frequency signal from the noise limiter diode to a sufficient degree and also it would not provide a sufciently low impedance in the short-circuiting path for the noise pulses through the limiter diode rectifier 12-13 across the output resistor 42. Also a large value of capacitance at 15 would not allow the D. C. voltage developed by the voltage doubler to follow fluctuations in the eld strength of a desiredsignal. In the present example also the demodulator or detector output resistor 42 may be of the order of 1/2 megohm.

From the foregoing description, it will be seen nthat,` in accordance with the invention, a de- 'modulator circuit of the diode-rectifier type is combined with a voltage-double rectifier to supply a controlling potential for a limiter diode whereby the entire output of the demodulator may be applied to the utilization circuit or succeeding ampliiier stage without resorting to tapping the output load resistor for the demodulator as has heretofore been the practice in providing automatic operation of the limiter With bias 0r controlling potential derived from the same source as the detected signal and thereby providing automatic level setting for the limiter. The circuit is also adapted for combining a plurality of tube functions in a minimum number of tubes.

We claim as our invention:

1. In a signal receiving system, the combination of a signal supply circuit, a demodu'lator, an output circuit for said demodulator, a limiter rectier connected to said output circuit, and voltage-doubler rectifier means for applying a control potential to the limiter rectifier comprising a voltage-doubler rectier and the demodulator connected in back-to-back relation across said circuit.

2. In a signal receiving system, the combinae,

tion of a signal rectiiier having an input circuit and an output circuit, a voltage doubler rectifier connected With said input circuit, a high impedance voltage doubler output connection from said signal rectier to said doubler rectifier, a limiter rectier connected to said first named output circuit, means providing a circuit for applying a control potential from said doubler output connection to said limiter rectiiier n opposition to signal potentials applied thereto,

and a lter in said last named circuit for controlling the rate of change of said control potential on said limiter rectifier.

3. In a radio-signal receiving system, the combination of a signal supply circuit, an audio irequency detector and voltage doubler comprising a pair of diode-rectier devices connected backto-back across said signal supply circuit, a voltage-doubler output circuit for said rectiiiers, a signal output circuit for one of said rectifiers including load resistance, a limiter diode rectier connected between said output circuits, a filter in said connection, and means for deriving signals from said output circuit through said limiter rectifier.

4. In a signal receiving system, a signal supply circuit, a diode detector circuit including a dioderectilier device, a voltage-doubler connection including said rst named diode-rectier device and a second diode-rectier device connected back-to-back with said first named device across said circuit, an output circuit for said voltagedoubler connection, a limiter-diode rectifier connected between the voltage-doubler output circuit and the detector circuit, means for deriving an audio-frequency signal from the detector circuit through the limiter-diode rectifler, and means for applying a received signal to said diode detector circuit and voltage-doubler connection therefor, whereby the limiter-diode rectier is controlled in accordance with signal strength variations.

WALDON P. BOLLINGER. NATHANIEL I. KOR/MAN. 

